Oh, the irony of an Islamic extremist named "Michael". 
Michael's relevance in Islam is fairly negligable compared to Christianity though. Jesus is also mentioned in the Quran. Doesn't mean that an Islamic terrorist going by the name of "Jesus" wouldn't be considered a bit awkward too.
The bottomline is that it's a primarily Christian name (and Jewish, being of Hebrew origin and all) and a primarily Christian figure in religion.
